Recently, solid-state lighting based on GaN semiconductors has made remarkable breakthroughs in efficiency. GaN-based diodes emit bright violet-blue light, which can be used to pump longer wavelength phosphors. The first white light emitting diodes (LEDs) became commercially available in 1997. These white LEDs can be obtained by combining a InGaN blue LED emitting at 465 nm with a broad-band yellow phosphor, e.g. (Y1-xGdx)3(Al1-yGay)5O12 (YAG:Ce). The variation of x and y can be used to produce a broad emission from 510 nm and 580 nm, leading to a high color rendering index. These white LEDs have efficiencies comparable to incandescent lights and are proving useful in a wide variety of niche lighting applications.
White light can be produced by a variety of other approaches, including color mixing of three LED emissions (e.g., combining discrete blue, green, and red LEDs) or the pumping of phosphors with a deep blue/UV LED or laser diode (LD). Nitride-based vertical cavity surface emitting lasers (VCSELs), coupled with phosphors optimized for violet or near-UV absorption, offer the greatest potential for high-efficiency solid-state lighting [D. A. Steigerwald, et al]. However, the problem lies in the unavailability of suitable RGB phosphors that are optimized for absorbing the near UV or violet emission from the LEDs or lasers. The red, green and blue phosphors that are currently used in conventional fluorescent lighting have been optimized for excitation by the UV emission from a mercury discharge, for which the characteristic wavelengths are 185 and 254 nm [G. Blasse, et al, 1994]. Hence, the challenge for the new generation of lighting based upon GaN lies in the development of novel families of phosphors that are optimized for excitations at longer wavelengths in the near UV (350–400 nm).
The current phosphor materials of choice for the solid-state lighting initiative are Y2O2S:Eu3+ for red, ZnS:(Cu+, Al3+) for green, and BaMgAl10O17:Eu2+ (BAM) for blue [M. Shinoya, et al]. Unfortunately, the red emission with Y2O2S:Eu3+ is inadequate in comparison with the green and blue phosphors, both in terms of its efficiency and its stability, so there is an urgent need to make superior red phosphors.